Downward flow reactors are used by the chemical and refining industries in a variety of processes, such as hydrotreating, hydrofinishing and hydrocracking. A typical downward flow reactor has a cylindrical outer wall with a catalyst bed positioned within the reactor. The catalyst bed generally rests on a catalyst support grid positioned horizontally within the reactor and spanning the complete diameter of the reactor. The catalyst support grid, together with the outer wall, cooperates to retain the catalyst or other particulate material in place. A distribution tray is positioned horizontally within the reactor at a location above the catalyst bed for evenly distributing process fluids onto the catalyst. The catalyst support grid, outer reactor wall and the distribution tray define the volume of the catalyst bed.
Multiple bed reactors are commonly used. They are formed by providing two or more such catalyst beds spaced along the longitudinal axis of the reactor. The region between successive catalyst beds defines an interbed mixing zone. When a reactor having more than one catalyst bed is used, reactant fluids are introduced into the reactor above the uppermost catalyst bed. The reactant fluids, which typically consist of both liquid and vapor phases, flow through the uppermost catalyst bed.
From the uppermost catalyst bed, unreacted reactant fluids and the related fluid products derived from interaction with the catalyst enter the interbed mixing zone. The interbed mixing zone typically includes a mixing chamber. This interbed mixing zone including a mixing chamber serves several purposes. First, the interbed mixing zone serves as a convenient place through which additional reactants and/or temperature-quenching materials can be introduced into the fluid products. In the reactor units described above, gas and liquid flow downward through multiple beds of solid catalyst. Because of this flow and the contact between the reactants and the catalyst(s), heat is released causing temperature to increase with distance down the bed. In many cases, cool hydrogen-rich gas is introduced between the beds to quench the temperature rise and replenish the hydrogen consumed by the reactions. Secondly, the interbed mixing zone provides a region for mixing the fluid products. Mixing the fluid products prior to reaction in lower catalyst beds ensures more uniform and efficient reactions. In addition, where catalytic reactions are exothermic and temperature control is a critical processing and safety element, mixing of the fluid products within the mixing chamber can be used to eliminate regions of locally high temperature within the fluid products.
The introduction and mixing of quench into the process gas and liquid must be carried out in the interbed space, which spans the full vessel diameter, but is often shorter than one vessel radius. Support beams, piping and other obstructions also occupy the interbed region so that unique hardware is required to perform efficient two-phase mixing in what amounts to limited volume.
Poor quench zone performance manifests itself in two ways. First, the quench zone fails to erase lateral temperature differences at the outlet of the preceding bed or, in the worst cases, amplifies them. An effective quench zone should be able to accept process fluids with 50 to 75 degree F. lateral temperature differences or higher and homogenize them sufficiently that differences do not exceed 5 degree F. at the following bed inlet. The second sign of poor performance is that inlet temperature differences following the quench zone increase as the rate of quench gas is raised. This indicates inadequate mixing of cooler gas with the hot process fluids.
Poor quench zone performance limits reactor operation in various ways. When interbed mixing is unable to erase temperature differences, these persist or grow as the process fluids move down the reactor. Hot spots in any bed lead to rapid deactivation of the catalyst in that region which shortens the total reactor cycle length. Product selectivities are typically poorer at higher temperatures; hot regions can cause color, viscosity and other qualities to be off-specification. Also, if the temperature at any point exceeds a certain value (typically 800 to 850 degree F.), the exothermic reactions may become self-accelerating leading to a runaway which can damage the catalyst, the vessel, or downstream equipment. Cognizant of these hazards, refiners operating with poor internal hardware must sacrifice yield or throughput to avoid these temperature limitations. With present day refinery economics dictating that hydroprocessing units operate at feed rates far exceeding design, optimum quench zone design is a valuable low-cost debottleneck.
A variety of multiple bed reactors and related mixing chambers have been previously described. For example, some mixing chambers are designed to impart rotational, radial, and/or turbulent flow to the fluids in the chamber. In others, a tortuous path is provided for improved mixing. Still other arrangements are designed to provide separate mixing of the vapor and the liquid phases of the fluids. An arrangement wherein two completely separate mixing chambers for imparting rotational flow individually to each phase prior to inter-phase mixing has also been described.
As stated above, the configuration of the mixing chamber must be designed to fit within the fixed volume of the interbed mixing zone while not substantially adversely affecting pressure drop within the reactor. Low pressure drops across the mixing zone are desirable to permit higher fluid flow rates within the reactor. The fixed volume of the interbed mixing zone is a result of design factors including the number of stages of catalyst required to achieve particular reaction characteristics and the desired flow rate through the reactor.
U.S. Pat. No. 4,836,989 describes a method for quench zone design. The essential feature of this design is the rotational flow created in the mixing volume, which increases fluid residence time and provides repeated contacting of liquid and gas from different sides of the reactor. This design is keyed to liquid mixing. More recent studies have shown it to be only a fair gas mixer. The trend to higher conversion and higher hydrogen circulation in fuels refining translates to gas/liquid ratios for which this design is not well suited. Height constrained units cannot be fitted with mixing chambers of the type described in this patent to the point that they are deep enough to effectively mix both the gas and liquid phases.
The interbed mixing system described in U.S. Pat. No. 5,462,719 offers some improvements over the design described above when gas mixing is paramount. This hardware is based again on a swirl chamber, but also includes at least three highly restrictive flow elements to enhance mixing, which such elements necessarily increase pressure drop. Like the previously described system, this quench zone mixes the gas and liquid at once in a single chamber.
Another system, which is disclosed in U.S. Pat. No. 6,180,068, referenced above, also provides enhanced mixing of quench gas and process fluids within the interbed space. This system employs separate mixing zones for each of two reactants permitting flexibility in mixing conditions while minimizing pressure drop as well as space and volume requirements. However, the efficiency of this device is sensitive to the degree of phase segregation achieved at the interbed inlet and thus may not perform as desired under all conditions and with respect to particular reactant characteristics.
The above and other known mixing systems generally suffer from the fact that there is insufficient space within the mixing chamber to promote intense two-phase mixing. Accordingly, there is a continued need to provide mixing systems that provide intense two-phase mixing. A preferred system also should provide sufficient volume for the vapor phase to mix separately from the liquid phase. Even while satisfying the above criteria, it is preferable that the designated mixing system minimizes the pressure drop within the reactor as well as permitting relatively easy retrofit with existing reactor systems.
A co-pending patent application filed by the assignee of the present invention describes the use of baffles in connection with the mixing system and the resulting process. According to the teachings of that invention and as further described in that patent application, at least one baffle comprising a substantially vertical continuous perimeter solid extrusion is attached to the underside of the collection tray. In a preferred embodiment of that invention, the diameter of the baffle is greater than the diameter of the mixing chamber outlet rim and smaller than the diameter defined by the spillway outlets. While that invention provides significant improvement in terms of mixing performance, it has been found that additional benefits and synergistic results can be obtained through the use of other and additional structural elements within the mixing system as described herein.
As can generally be surmised from the above discussion, among other things, there is a deficiency in the prior art with respect to the mixing of the gas phase. Prior art interbed mix zone designs typically feature a common inlet to the mixing chamber for both gas and liquid. This gives the beneficial result that the liquid is propelled into the mixing chamber at a high velocity determined by both the gas and liquid flow rates. This inlet velocity typically imparts enough kinetic energy to the liquid to guarantee significant rotation in the mixing chamber. By contrast, the gas, being of lower density, dissipates the entering kinetic energy and departs from the desired rotational motion more quickly. As a result, gas phase mixing in prior art systems has been less than ideal.